In automotive vehicles control circuits comprise printed circuit boards containing a harness connector, a number of components and traces or circuit paths interconnecting the components and the connector. It can occur for a particular circuit board design that radio frequency currents can be developed and carried in the traces to cause an electromagnetic field to emanate. Depending on the circuit design and the strength of the field, it can cause undesirable radio interference. This is especially true if a strong field occurs in the region of the harness connector and results in rf currents in the harness which acts as an antenna to radiate interference signals. Conversely, the harness can pick up rf energy from another source and carry rf current to the circuit board which may have adverse effects on the circuit. Such undesirable current also results in field emanation from the circuit board.
It is desirable to measure the field emanating from the printed circuit board in sufficient detail to establish the field pattern and provide an indication of where the fields are on the board and consequently what elements or traces carry the rf current. With this knowledge, it is then possible to correct any problem, say by adding a filter at an appropriate place in the circuit or even redesign the circuit to prevent the objectionable radiation. Examination of prototype circuit boards by analyzing the electromagnetic fields adjacent the board allows circuit refinement before production begins. Experience of a circuit designer with such analysis gives insight into the effect of circuit path configuration on electromagnetic emission so that initial designs can be made which avoid problems of rf radiation and susceptibility to rf fields.
It is already known to measure and map fields radiated from circuit boards. One known electromagnetic emissions scanner comprises a circuit board carrying a rectangular array of 1,280 H-field loop probes having a fixed spacing of 7.6 mm between probes. The probes are oriented in two mutually perpendicular directions to sense fields from conductors running in any direction. The scanner is positioned adjacent a printed circuit board under test so that each probe senses the magnetic field at its location. The probes are sequentially addressed by a computer to retrieve field data and a receiver or a spectral analyzer collects the data and displays the information in color according to frequency and field strength to thus map the field. This tool has the drawbacks of high cost, due to the complex scanner with many probes, and low resolution due to the large spacing between the probes. Since many circuit boards have high density circuit paths, the low resolution limits the scanner usefulness.